The invention relates to a method for producing glass panes, in particular glass panes with a thickness of less than 3 mm, by drawing a thin glass ribbon vertically downward, in which a glass melt is conveyed from a tank furnace, through an inlet, and to a drawing tank with a nozzle system that has at least one debiteuse. The invention also relates to a device comprising a tank furnace, a homogenization system, an inlet and a drawing tank, wherein the drawing tank has a nozzle system having at least one slit nozzle.
Thin glass panes of this kind are used as substrate glass in electronic devices, for example displays (portable telephones, flat screens, etc.) and digital mass storage devices of computers. Therefore extremely high demands are placed on the internal quality of the glass that is essentially determined by bubbles and inclusions, on the cleanness, on the quality of the surface geometry that is essentially determined by the fine corrugations (waviness) and by deviations from flatness (warp), on the breaking strength, and possibly also on the low weight.
When the glasses are used as substrate glass for displays, clients subject them to thermal manufacturing processes at temperatures that approach the transition temperature of glass. The dimensional stability of the glass substrates must be maintained in the course of this. Therefore special glasses or even glass ceramics with increased glass transition temperatures are used as substrate glasses, most of which have an increased tendency to crystallize and an atypical viscosity curve as a function of the temperature. These glasses consequently require higher processing temperatures than standard glasses.
Since these applications are mass produced products, it is necessary to manufacture the glass panes as inexpensively as possible. It is thus desirable to achieve a high degree of process stability with short set-up times and down times, with high throughputs and low waste, which are caused by glass defects and in the border region. In addition, it is necessary to largely fulfill the client requirements with regard to the cleanness of the surface and the quality of the surface geometry, thus allowing the high-cost processing steps of finishing work, e.g. grinding and polishing, to be reduced or eliminated.
In the drawing methods that have been known up to this point, the above-mentioned requirements with regard to the product quality and economy are only partially fulfilled.
Among the manufacturing methods, a distinction is drawn between drawing methods with and without a debiteuse.
In the drawing method without a debiteuse, which is described for example in U.S. Pat. No. 3,338,696, a trough is used into which the glass melt is conveyed. The glass melt runs over the upper edge of the trough walls and travels downward along the outsides of the wedge-shaped trough. At the vertex point, the glass films thus produced flow together and are drawn downward. The ceramic trough is clad with platinum to reduce corrosion.
In this method, the total throughput is essentially determined by the inlet between the tank furnace and the trough. The so-called linear throughput, which is understood to be the throughput per unit of length lateral to the drawing direction of the glass ribbon, is adjusted by means of the glass flow in the trough, the glass level in the vicinity of the overflow lip of the trough, the geometry of the overflow lip, and the viscosity of the glass. A very exact temperature regulation is required, which must be to within 0.1° C.
Format or throughput changes require the geometry of the drawing trough to be adapted, in particular for the glass flow in the trough. Since it takes up to a week or more to start up a new drawing trough, the desired flexibility of production is only achieved to a limited degree.
The corrosion of the trough edges cannot be compensated for by tilting or stretching the trough. The trough must be replaced and the process must then be started up again.
In order to produce a wide glass ribbon with narrow borders, the glass ribbon must be stretched lateral to the ribbon direction, by means of so-called border rollers in the edge region. The border rollers increase the complexity of the manufacturing process.
Among the drawing methods with a slit nozzle, a distinction is drawn between those with and without draw bars or spreaders.
In the method with a slit nozzle, but without a draw bar described for example in SU 617,390, the glass from the refiner or conditioner runs over the opposing walls on both sides of an overflow weir made of a fireproof material. The two glass films thus produced flow together above a nozzle and are then drawn downward. The throughput is regulated by means of the glass level at the overflow lips. This can occur either by changing the glass level in the refiner or by more deeply immersing the overflow block.
The method with a slit nozzle but without a draw bar cannot fulfill the increased demands for surface quality, in particular with regard to fine corrugations (waviness). Because of the short dwell time of the glass in the onion region and because of the high viscosity of the glass, the irregularities do not heal.
It is also impermissible to exceed certain processing temperatures due to the increased instabilities in the onion region when the glass melt falls below critical viscosities. Consequently, the known drawing methods without a draw bar cannot be used to produce special glasses with an increased tendency to crystallize.
The disadvantages described above are partially avoided through the use of a slit nozzle with an internal draw bar.
U.S. Pat. No. 1,759,229 describes a drawing method for producing flat glass, in which glass flows through a slit nozzle that is provided in the bottom of a refiner or conditioner, onto a spreader with a rhomboid cross section, and is drawn downward over this spreader. Underneath the slit nozzle, the spreader is fitted into a recess that widens out toward the bottom. The glass bath depth of the refiner or conditioner increases toward the ends of the slit nozzle. Both the nozzle and the spreader can be contoured. It is essential here that the nozzle slit widens out toward the edges and that the alignable spreader that is used can be bowed upward in the middle.
The lower, wedge-shaped part of the spreader is provided with an enclosed region that can contain heating or cooling elements extending lateral to the drawing direction. The temperatures of the spreader can be adjusted by means of flows of temperature control mediums.
The glass flow is influenced by the geometry of the nozzle and spreader, by the temperature level and profile in the refiner or conditioner, and by the position of the spreader in relation to the nozzle. Among other tasks, the spreader must homogenize the temperatures of the glass lateral to the drawing direction so that flat glass is produced with the predetermined dimensions and a uniform appearance.
DE-AS 15 96 484 has disclosed a device, which includes a homogenizing receptacle that is connected to a drawing furnace by means of a closed, heatable channel. The drawing furnace is equipped with a nozzle, which is provided with a slit made of platinum at the bottom. Underneath the nozzle, a draw bar is provided in the form of a vertically situated plate. The glass melt emerging from the nozzle travels downward on both sides of the draw bar and comes together at the bottom end to form a glass ribbon.
For heat dissipation from the inside, bores are provided in the draw bar, through which coolant can be conveyed. In addition, cooling bodies are attached to the outside of the draw bar at the bottom. The height of the draw bar can be adjusted by means of adjusting screws.
JP 2-217,327 has disclosed a device for producing flat glass in which the glass is drawn downward through a slit that is provided in the bottom of a feeder or a refiner. The slit nozzle can be heated indirectly. The glass flow is shut off with the aid of a plunger disposed over the slit. In order to stabilize the glass ribbon, the debiteuse has a plate-shaped internal draw bar disposed in it, which is secured at the sides and can be adjusted in position, whose upper section protrudes into the debiteuse and whose lower part is encapsulated.
U.S. Pat. No. 1,829,639 describes a method and a device with which the total throughput and the linear throughput are adjusted only by means of the geometry of the nozzle system, the viscosity of the glass melt in the vicinity of the nozzle, and the pressure of the glass melt through the nozzle. Upon emerging from the nozzle, the glass melt has only a slight excess pressure due to the open storage system with a low glass fill level. The essential disadvantage of this device and this method lies in the coupling of the total throughput and the linear throughput.
The known methods with a slit nozzle and draw bar cannot achieve the excellent surface qualities that can be achieved with the known drawing methods without the slit nozzle. Particularly at high linear throughputs and high processing temperatures, the dwell times of the glass film on the draw bar are not sufficient to heal the waviness of the surface, which is caused by the wetting of the nozzle in the vicinity of the breaking edge.
In the known methods with the slit nozzle and without a draw bar, the adjustments to the total throughput and the adjustment of the linear throughput (thickness distribution lateral to the drawing direction) are coupled to each other.
In format or throughput changes, the nozzle geometry and the temperature control in the vicinity of the nozzle must be constantly renewed in order to be able to make adjustments in a generally empirical fashion. The starting processes last a long time and the desired flexibility of production is only achieved to a limited degree.
In order to adjust the specified thickness distribution, defined temperature profiles must be set lateral to the drawing direction in the vicinity of the nozzle. The temperature profiles imparted to the glass ribbon can only be partially balanced up to the fine annealing zone. This can lead to an impermissible deformation of the glass ribbon (warp) during the cooling to room temperature.